The core of an aircraft wing is a section called a wing box. The wing box is fared into the aircraft fuselage and runs from the root towards the tip of the wing. The wing box provides the central sections of the upper and lower aerofoil surfaces for the wing in addition to attachment points for engines and control surfaces such as ailerons, trim flaps and airbrakes. The aerofoil surfaces of the wing box are provided by panels referred to as wing cover panels. The wing covers panels have a number of structural elements called stringers attached to their inner surfaces and which run within the wing box structure from the root towards the tip. The stringers are arranged to provide the necessary structural stability and integrity to enable the wing covers to cope with the operational loads on the wing.
When building aircraft structures, one of many considerations is the balance between weight and strength. In other words, structures need to be as light as possible while providing the necessary strength and structural integrity to perform properly under operational loads. The loads on a wing vary along its span and typically reduce towards the wing tip. Therefore, stringers can be built to have strength that varies along their length, thus reducing their overall weight. Stringers are commonly created by joining or splicing stringer sections, each section having weight characteristics tailored to its expected loading. Alternatively, a single stringer can be provided and machined to a varying degree along the span so as to reduce unnecessary weight. The finished stringer is then typically fixed to the interior surface of a wing cover panel within the wing box.
A problem with splicing or joining sections of stringer is that it adds extra weight and complexity to the wing box. A problem with machining a single stringer is that the amount by which the stringer can be modified is limited by the envelope of the stringer cross section. If significant machining is desired then a larger envelope is required. This increases the cost of the stringer due to the greater the amount of waste material produced. Also, fixing a stringer to the wing cover panel requires a large number of holes to be drilled in both the stinger and wing cover for the appropriate fixing. These holes are potential crack initiation sites and the fixings themselves increase the overall weight of the wing box.